I won't claim to be in the same league of shooters that you have gotten advice from, but I will say that I've learned more from my .223s than any other rifle (22rf could be the exception).
Very common and cheap brass, tons of good bullets from under 40 to over 80 grains, easy to reload, easy to shoot, easy to spot your hits through the scope at all ranges. Shooting 50's in the wind is pretty instructive....
If I went that route it would be a good excuse to get an AR... : )
